This application claims the benefit of Korean Application No. 00-10875 filed Mar. 4, 2000, in the Korean Patent Office, the disclosure of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to an optical pickup for high-density recording/reproduction compatible for optical discs having different formats, and more particularly, to an optical pickup for high-density recording/reproduction adopting a single objective lens, which is compatible for optical discs having different formats, and corrects for chromatic aberration caused by different wavelengths of light and/or spherical aberration due to differences in thickness of optical discs.
2. Description of the Related Art
Optical pickups are a device for recording information on or reproducing information from an optical disc by focusing a laser beam on the optical disc with an objective lens. The recording and reproduction capacity is determined by the size of a focused spot. The size of the focused spot is related with a wavelength (xcex) of the laser beam, and a numerical aperture (NA) of the objective lens, as shown in equation (1):
size of focused spot xe2x88x9d xcex/NAxe2x80x83xe2x80x83(1)
For a higher recording density of about 15 gigabytes or more, the size of the spot being focused on the optical disc must be further reduced. To form a small spot for high-density recording, as can be inferred from equation (1), it is essential to adopt a blue laser as a light source, which emits light having a short wavelength of about 410 nm, and an objective lens having an NA of 0.6 or more.
On the other hand, coma W31, which occurs due to tilting of the optical disc, is associated with a tilt angle (xcex8) of the information recording surface of the disc with respect to an optical axis, a refractive index (n) of the disc substrate, the thickness (d) of the disc substrate, and the NA of the objective lens, as expressed by equation (2):                               W          31                =                                            -                              d                2                                      ·                                                                                n                    2                                    ⁡                                      (                                                                  n                        2                                            -                      1                                        )                                                  ⁢                sin                ⁢                                  xe2x80x83                                ⁢                θcos                ⁢                                  xe2x80x83                                ⁢                θ                                                              (                                                            n                      2                                        -                                                                  sin                        2                                            ⁢                      θ                                                        )                                                  5                  /                  2                                                              ⁢                      NA            3                                              (        2        )            
To ensure tolerance with respect to the tilt of disc for high density recording, there is a tendency of reducing the thickness (d) of the disc substrate. For example, compact discs (CDs) have a thickness of 1.2 mm and digital versatile discs (DVDs) have a thickness of 0.6 mm. Also, there is a high possibility that the thickness of future generation DVD family media (so-called high-definition (HD)-DVDs), which are recently being developed, is determined to be 0.6 mm or less.
Optical pickups for high-density recording/reproduction in/from future generation DVDs adopt a light source which emits a blue laser beam, and an objective lens optimized to be suitable for the blue laser beam and the thickness of a future generation DVD substrate.
For compatibility with existing discs, such as DVDs, the optical pickup for high-density recording and reproduction needs another light source which emits a red laser beam. The reason why both blue and red light sources are adopted in the optical pickup for future generation DVDs is for compatibility with DVD-recordable (DVD-R) and multi-layered DVDs, which have a low reflectivity with respect to blue light.
The objective lens of the optical pickup for future generation DVDs is designed to be suitable for blue light and the thickness of a future generation DVD substrate. Thus, when a DVD is adopted as a recording medium, a red light spot focused on the recording surface of the DVD by the objective lens includes chromatic aberration due to a difference in wavelengths of red and blue light. In addition, when the thickness of a future generation DVD substrate used is different from that of a DVD substrate, spherical aberration caused by the thickness difference of the discs occurs.
FIG. 1 illustrates the optical path difference (OPDrms) in an optical pickup adopting an objective lens designed exclusively for 405 nm light with respect to wavelength variations of light incident on the objective lens. In FIG. 1, OPDrms refers to the amount of aberration in a light spot focused by the objective lens and is expressed in wavelengths.
As shown in FIG. 1, when 405 nm light is incident on the objective lens, almost no aberration occurs, so that the OPDrms at 405 nm is close to zero. In contrast, when 650 nm light is incident on the objective lens, the OPDrms at a wavelength of 650 nm becomes 0.15xcex due to increased aberration.
Thus, in consideration of a standard aberration allowance, i.e., OPDrms=0.07xcex, in the related field, the optical pickup for future generation DVDs, which is designed to focus 650 nm light with the objective lens optimized for 405 nm light, is not compatible with DVDs. In other words, for the compatibility with DVDs, the optical pickup for future generation DVDs needs to correct for chromatic and/or spherical aberration mentioned previously.
Referring to FIG. 2, a conventional aberration correcting apparatus includes an objective lens 3 which focuses an incident beam and a condensing lens 5 which further condenses the beam focused by the objective lens 3 to form a light spot on an optical disc 1. As shown in FIG. 3, a distance between the condensing lens 5 and the objective lens 3 is adjusted according to thickness variations (xcex94d) of the optical disc substrate 1 and wavelength variations of light used, such that aberration is corrected.
However, as for such a conventional aberration correcting apparatus adopting two lenses, the condensing lens 5 and the objective lens 3, assembling the two lenses is complicated. Also, the objective lens 3 and the condensing lens 5 must be actuated for both tracking and focusing control, and adjusting the distance between the objective lens 3 and the condensing lens 5, so the structure of the entire actuator becomes complicated.
To solve the above problems, it is an object of the present invention to provide an optical pickup for high-density recording/reproduction adopting a single objective lens, in which chromatic aberration caused by different wavelengths of light and/or spherical aberration due to thickness variations of optical discs is corrected, and thus the optical pickup is compatible for optical discs having different formats.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and, in part, will be obvious from the description, or may be learned by practice of the invention.
To achieve the above object and other objects of the present invention, there is provided an optical pickup compatible for optical discs having different formats. A first light source emits a beam having a relatively short wavelength suitable for a first optical disc with a format and a second light source emits a beam having a relatively long wavelength suitable for a second optical disc with another format. An objective lens, designed to be suitable for the first optical disc and the wavelength of the beam emitted from the first light source, focuses an incident beam to form a light spot on a corresponding optical disc. An optical path changing system alters a traveling path of the beams emitted from the first and second light sources. A first photodetector receives and photoelectrically converts the beam reflected from a corresponding optical disc and passed through the objective lens after having been emitted from the first light source. A first signal processing unit detects a reproduction signal of the first optical disc from the signal output from the first photodetector. A first light splitter splits an incident beam into at least two beams including first and second beams to be focused as a main light spot and a sub-light spot, respectively, on the second optical disc after having been emitted from the second light source. A second photodetector comprises first and second light receiving portions which respectively receive and photoelectrically convert the first and second beams reflected from the corresponding optical disc after having been emitted from the second light source. A second signal processing unit processes the electrical signals output from the first and second light receiving portions of the second photodetector to correct for chromatic aberration caused by a difference in the wavelengths of the beams emitted from the first and second light sources and/or spherical aberration caused by a thickness difference between the first and second optical discs, thereby detecting a reproduction signal of the second optical disc.
It is preferable that the first light source emits blue light and the second light source emits red light. It is preferable that the first light splitter is a holographic optical element (HOE) which causes a predetermined amount of spherical aberration only to the second beam, such that the second beam further includes spherical aberration relative to the first beam. In this case, the second signal processing unit may further comprise a delay between an output of the first and/or second light receiving portions and at least one input of the second processing unit, to delay one of the signals output from the first and second light receiving portions to match the phases of the electrical signals.
It is preferable that the first light splitter is a polarization holographic optical element (HOE) which generates a first beam having one polarized component and a second beam having another polarized component, and which causes a predetermined amount of spherical aberration only to the second beam having the another polarized component, such that the second beam further includes spherical aberration relative to the first beam. Preferably, the optical pickup further comprises a polarization beam splitter which transmits or reflects the first and second beams reflected from the second optical disc according to the polarization of the first and second beams, wherein the first and second light receiving portions of the second photodetector are arranged to separately receive the first and second beams having different polarized components split by the polarization beam splitter.
Assuming that a main reproduction signal from the main light spot, which has been received and photoelectrically converted by the first light receiving portion, is Sm, a sub-reproduction signal from the sub-light spot, which has been received and photoelectrically converted by the second light receiving portion, is Ssub, and k is a gain factor, the second signal processing unit preferably process the signals output from the first and second light receiving portions using equation (3) below, to output a final reproduction signal S from which chromatic aberration caused by different wavelengths of the beams emitted from the first and second light sources, and/or spherical aberration caused by thickness difference between the first and second optical discs, are corrected:
S=Sm+k(Smxe2x88x92Ssub)xe2x80x83xe2x80x83(3)